The combustion of sulfur-containing fuels in power plants or industrial boilers produces flue gases that contain sulfur dioxide (SO2). Environmental regulations require that these combustion sources control their emissions of SO2. Conventional control technologies include wet scrubbers, spray dryers and dry sorbent injection.
Wet scrubbers are large absorber vessels that contact the flue gas with a recirculated slurry or liquor. The flue gases are quenched in the vessel and fully saturated with moisture, allowing SO2 to be absorbed into the scrubbing liquid. Subsequently the absorbed SO2 is precipitated to produce a solid sludge, or conversely discharged as a wastewater solution. The process can achieve very high SO2 removals (>95%) and is generally applied to flue gas steams with high SO2 concentrations. Wet scrubbers have several disadvantages including very high capital cost, large footprint, high flue gas pressure drop, and costly waste treatment and disposal. Wet scrubbers for SO2 control were originally developed in the 1970's and 1980's, and are now a conventional and mature technology.
Spray dryers are also large absorber vessels that contact the flue gas with a slurry or liquor. However, in a spray dryer the flue gas is only partially quenched or saturated. A slurry mixture of alkaline solids and liquid is injected into the vessel, where it is atomized to produce fine droplets. SO2 is absorbed into the droplets where it reacts with solids in the slurry before the droplet fully evaporates. The resulting dried solids containing the SO2 are then captured in a particulate control device downstream of the spray dryer vessel, typically a new fabric filter or baghouse. The dried solids are captured with other particulate in the flue gas, including fly ash, and typically disposed of in a landfill. The process can achieve moderately high levels of SO2 removal (70-90%), but typically less than a wet scrubber. As a result, the process is typically applied to flue gas streams with moderate SO2 concentrations. Spray dryers have several disadvantages including very high capital cost, large footprint, high flue gas pressure drop, and high reagent and operating costs. Spray dryers used for SO2 control were originally developed in the 1980's, and are now a conventional and mature technology.
Dry sorbent injection (DSI) relies on the injection of very fine, dry alkaline solid powders into existing flue gas ductwork to contact the flue gases. The injected powders typically include hydrated lime, sodium bicarbonate, and sodium sesquicarbonate (sometimes referred to as trona). These sorbents, or reagents, are more expensive than those used in wet scrubbers or spray dryers. The sorbents react with SO2 in the flue gas, with the resulting reaction products collected in a downstream particulate collection device, along with fly ash in the flue gas. SO2 removal for DSI is typically the lowest (30-80%) of the three processes described, with the higher levels requiring very high reagent injection rates. Dry sorbent injection has several disadvantages including only modest SO2 removal, high sorbent and operating costs, and adverse impacts on downstream particulate collection equipment performance, in some cases.
There are a number of patents related to control of SO2 emissions from combustion gases. However, the technology claimed in most patents is well known in the art and the patents have long-ago expired. In addition, most of the patents relate to wet scrubber and spray dryer processes and are very narrowly directed to specific aspects of those technologies.
More recently, several patents have been issued for SO2 control using dry sorbent injection. Some of the patents recently issued in the United States include: U.S. Pat. No. 7,531,154 B2 entitled “Method of removing sulfur dioxide from a flue gas stream,” U.S. Pat. No. 6,303,083 B1 entitled Method and system for SO2 and SO3 control by dry sorbent/reagent injection and wet scrubbing,” and U.S. Pat. No. 8,828,340 B2 entitled “Dry sorbent injection during steady-state conditions in dry scrubber.” However, these patents are all directed to the injection of dry sorbents, and thus are different from the novel features described herein.
Another patent related to the injection of aqueous solutions for acid gas control from flue gases is U.S. Pat. No. 6,803,025 B2. However, this patent describes a process for the removal of SO3 and H2SO4 from flue gases, but not the removal of SO2. In typical combustion gases, the concentration of SO3/H2SO4 is only one to two percent of the SO2 concentration. However, in boilers that burn high-sulfur fuels (>1%), the resulting SO3H2SO4 concentration (10-50 ppm) is sufficient to contribute to acid corrosion of equipment in the flue gas path, and other adverse impacts. As a result, the patented process is specifically applied to boilers that burn these higher-sulfur fuels, and for the specific removal of SO3/H2SO4.
There is a need, therefore, for a method and system that does not have the high capital cost, large footprint, high water consumption, high pressure drop, and sludge and wastewater production that is costly to treat and dispose of as with wet scrubbers, high capital cost, large footprint, high water consumption, and high reagent operating costs as with spray dryer, or the limited SO2 removal and high reagent operating costs as with dry sorbent injection.